Composite utility poles have recently been introduced to offer various advantages over other types of poles historically used or more recently introduced. Such composite utility poles are hollow structures having a polygonal-shaped outer surface and an inner channel fabricated of rovings of fibers located in a zero degree orientation and layers of fibers or mats embedded in resin. The poles are ultraviolet light resistant, corrosion resistant, resistant to bugs, birds and the like, and are not subject to rot.
In such composite pole structures, it is desired to minimize the amount of material required for the fabrication of the pole while still maintaining accurate strength, as weight and cost are both highly dependent on the amount of each material used.
In normal use, utility poles may be subject to various forces, some of which are relatively constant, some of which are dependent upon and vary with the environment and some of which will vary dependent upon the position and application of the pole in the system. By way of example, poles are normally required to carry their own weight, the weight of one or more cross-arms and the weight of the wires supported thereby. Additionally, they may encounter the weight of a transformer or other parts of the distribution system. Variable forces include, the weight of birds perched on the wires, and the unequal tension in the wires because of the birds, windage, snow and ice on the pole, cross-arms, wires and any other components supported thereby such as by way of example, transformers. Other forces that may be encountered by utility poles include side forces arising from the fact that utility poles are not always placed directly inline with each other. By way of example, utility poles positioned along a curving street will similarly be positioned in a curved arc so that the tension on the wires together with any increased tension due to birds on the wires, etc. will provide a side force adjacent the top of the pole, tending to pull the top of the pole toward the center of the curve. Since horizontal forces at the top of a pole create large bending moments along the pole and particularly at the base of the pole, it is particularly important that such composite poles have adequate resistance to such bending moments without failure of the structural integrity of the pole. As a further illustration of a situation wherein high horizontal forces, may be exerted on a pole, consider a windstorm situation wherein trees fall across power lines (or phone lines). Preferably, the wires will fail, but the poles will be left standing. Further, however, it is important that with wires on one side of the pole being severed, the tension in the wires on the other side of the pole, given windage, perhaps ice accumulation, etc. will not cause a failure in the pole, as otherwise a domino effect may be encountered where each pole in a row would fail one after another.
Because of the aspect ratio of a typical utility pole is high, the high bending moments encountered along the pole and particularly adjacent the bottom of the pole normally impose more severe structural requirements on the pole than are imposed by the weight of the structure, wire, birds, etc. that must be supported by the pole through the compressive loads imposed thereon.